The current project is directed toward elucidating the structure and function of Kirsten and Harvey murine sarcoma virus genomes. Both are recombinant viruses and their genome structure has been delineated. A viral gene product, the p21 protein, encoded by these two viruses has been identified in non-producer cells transformed by these viruses. The p21 appears to be required for the maintenance of cell transformation induced by these sarcoma viruses as demonstrated by its enhanced thermolability in a viral mutant temperature-sensitive for the maintenance of transformation.